Born Alexander Anderson McSween, in Canada, McSween was for a time in his youth a preacher. Afterwards he attended law school for one year, in St. Louis, Missouri before moving to Kansas, then to New Mexico Territory. He had married Susan Hummer in 1873. After he arrived in Lincoln County, New Mexico, McSween worked for Murphy-Dolan as an attorney, then left them to work for John Tunstall, with whom he became close friends and by 1876, business partners.
When the trouble between the two factions began, Murphy-Dolan accused McSween of embezzlement, and they hired gunmen from the Evans and Kinney gangs to rustle Tunstall's cattle and harass him. McSween-Tunstall hired gunmen to protect them and their properties, including Billy the Kid, Dick Brewer, George Coe, and Frank Coe, among others. On February 18, 1878, Tunstall was murdered by William Morton, Jesse Evans and Tom Hill.
The Lincoln County Regulators were formed, and on March 6, 1878, the Regulators arrested Frank Baker and William Morton, and executed them. On April 1, 1878, Billy the Kid, Jim French, Frank McNab, John Middleton, Fred Waite, Henry Brown, entered Lincoln and ambushed and killed Sheriff Brady and Deputy Hindman. Deputy Billy Matthews returned fire during the attack, wounding Billy the Kid and Jim French, but the wounds were minor.
On April 4, 1878, there was a gun battle at Blazer's Mill between Buckshot Roberts, a bounty hunter working for Dolan, and the Regulators. Roberts and Regulator Dick Brewer were killed, Middleton was badly wounded, the Kid was grazed by a bullet, and George Coe had his trigger finger shot off. On April 18, Billy the Kid, John Middleton, Fred Waite and Henry Newton Brown were indicted for the murder of Sheriff Brady, while Dolan, Jesse Evans, Matthews and others were indicted for the murder of Tunstall. On May 15, 1878, Manuel Segovia, the cowboy who had killed Frank McNab, was captured.
On July 15, 1878, the Regulators were surrounded in Lincoln at the McSween home, along with McSween and his law partner, Harvey Morris. Facing them were the Dolan/Murphy/Seven Rivers cowboys, led by Sheriff George Peppin. On July 19, after numerous exchanges of gunfire over a four-day period, the house was set afire. As the flames spread and night fell, Susan McSween was granted safe passage out of the house while the men inside continued to fight the fire.